Fuel cells are electrochemical energy conversion devices considered as a possible alternative to internal combustion engines. Fuel cells convert a hydrogen containing fuel such as methanol or hydrogen to electrical energy by an oxidation reaction. A by-product of this reaction is water. Adequate output voltage entails the assembly of multiple fuel cells, connected in series, into fuel cell stacks.
One type of fuel cell comprises a solid polymer electrolyte (SPE) membrane, such as a sulfonated fluorinated polymer membrane material known as Nafion, which provides ion exchange between cathode and anode electrodes. Various configurations of SPE fuel cells as well as methods for their preparation have been described. See e.g. U.S. Pat. No. 4,469,579; U.S. Pat. No. 4,826,554; U.S. Pat. No. 5,211,984; U.S. Pat. No. 5,272,017; U.S. Pat. No. 5,316,871; U.S. Pat. No. 5,399,184; U.S. Pat. No. 5,472,799; U.S. Pat. No. 5,474,857; and U.S. Pat. No. 5,702,755.
A membrane electrode assembly or MEA for the fuel cell is formed by bonding of a cathode catalyst, the solid polymer electrolyte (SPE) layer and an anode. A porous conductive carbon cloth is placed in between each MEA and a separating element. A fuel cell stack of fuel cells connected in series is made by repeating the sequence described above so that a multiplicity of single cells forms the stack.
The separating element serves to transport reactants and products to and from the fuel cell and thus is also often referred to as a flow-plate. The separating element also manages heat output of the fuel cell, by transferring or distributing heat generated by the fuel cell to its surroundings.
Typically, the separating element comprises a separator sandwich formed by placing an anode plate over a cathode plate in the following sequence. The front face of the anode plate serves as the anode separator flow field while the rear face of the anode plate serves as the anode separator face with transfer cavities. The rear face of the anode plate is adjacent to the separator face with transfer cavities of the rear face of the cathode plate. The front face of the cathode plate serves as the cathode separator flow field. Thus, the posterior or rear face of one flow directing separator plate for the anodic process is placed in contact with the posterior or rear face of the corresponding cathodic separator plate. This assembly forms the integral separator sandwich in the conventional cell. Apertures and orifices on the anterior surface of the cathode flow plate and the anode flow plate are arranged so that the appropriate reactants are fed to either the anode surface or the cathode surface via cavities enclosed by the plane surface of the opposing separator plate element. Leakage is prevented by polymeric seals placed in grooves surrounding these cavities.
Since properties of the cathodic and anodic reactants are different, the flow pattern and channel configuration and design are adapted to the particular material being transported to the MEA via the channels in the separator plate. A system of apertures in the separator plates form a common supply channel for each of the reactants and traverses the stack, supplying reactants to each fuel cell via apertures arranged on the appropriate separator plate faces. Thus, an oxidant is supplied to the cathode where reduction occurs and a hydrogen containing fuel such as hydrogen or methanol is supplied to the anode where oxidation occurs.
Separating elements are typically manufactured from conducting carbon composites, such as that supplied as SIGRACET Bipolar Plate BMA 5 by SGL Carbon, Meitingen, Federal Republic of Germany.
The use of separating elements has disadvantages. The foremost is the undesirable replication of parts and the undesirable increase of the volume of the stack and its weight as it is difficult to manufacture very thin plates in the approved materials without steeply increasing quality defects. There is a duplication of elements having very similar functions where differentiation is not required. There is also increased electrical resistance in the cell thus affecting the heat loss due to resistive power dissipation and giving uneven power distribution and reduced output. Further, contact between the plates of the separating elements can deteriorate considerably after extended cell cycling. This deterioration is believed to be caused by chemical and tribiological changes in the contact layer between anodic and cathodic separator plates.
Attempts have been made to address these issues.
U.S. Pat. No. 6,503,653 discloses a bipolar plate assembly for a PEM fuel cell having a serpentine flow field formed on one side and an interdigitated flow field formed on the opposite side. Thus, in this assembly, a single plate serves as both the anode current collector and a cathode current collector of adjacent fuel cells. This bipolar plate assembly further comprises a staggered seal arrangement to direct gaseous reactant flow through the fuel cell such that the seal thickness is maximized while the repeat distance between adjacent fuel cells is minimized.
U.S. Pat. No. 6,500,580 discloses a fluid flow plate for a fuel cell including a first face and a fluid manifold opening for receiving a fluid and at least one flow channel defined within the first face for distributing a reactant in the fuel cell. A dive through hole is defined in and extends through the fluid flow plate. The dive through hole is fluidly connected to the fluid manifold opening by an inlet channel, defined within an opposite face of the plate. The dive through hole and the inlet channel facilitate transmission of a portion of the fluid to the flow channel. A groove, adapted to receive a sealing member, is also defined within the first face and/or the opposite face. The sealing member may comprise a gasket which seals the respective fluid manifolds, thereby preventing leaking of fluid.
WO 01/71836 discloses a plate assembly formed by two separator plates positioned back to back. The separator plates are fitted with fluid channels of grooves, which collectively form what is conventionally termed as the flow field. In this plate assembly, the grooves open out into continuous inlet and outlet openings. A cover, termed a bridge, is placed in the outlet where it opens out and lies flush with the groove surface in order to ensure that a fluid seal is maintained around the flow field domain. As the bridge thickness is less than the separator plate thickness, access for fluid to the flow field is attained via the aperture formed between the anterior face of the bridge and the anterior surface of the corresponding separator plate. Thus, a cavity with a by-pass under the bridge is formed to give fluid access while at the same time a flush sealing surface is presented on the flow-field surface so that efficient sealing can be achieved. A continuous channel or manifold system is formed collectively by the inlets upon assembly of the plates into a stack. All fuel cells in the stack can be adequately supplied with reactants without breaching the seal enclosing the electrochemical cell.